[clang] [ast] CXXRecordDecl::getVisibleConversionFunctions() could be const

The function and its called static helpers don't modify the received
CXXRecordDecl arguments at all as the method's result is put into an
output parameter. Thus they can be const which allows for neatly
grabbing the conversion methods in a context where we only have a const
ASTUnit at hand.

Differential Revision: https://reviews.llvm.org/D71805